# Issued: Sat, Feb 18, 2017 at 7AM

## Expires: Sun, Feb 19, 2017

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# Below 2,500ft Low

1. Low
2. Moderate
3. Considerable
4. High
5. Extreme

## Problem Details

AVALANCHE PROBLEM: PERSISTENT SLAB

Shallow buried near surface facets, and weak, basal facets, have teamed together to provide a complex persistent slab problem. Natural avalanches are unlikely, however, human triggered avalanches are possible, in specific, leeward, wind loaded locations.

Old problem: The deep, basal facets, will be difficult and stubborn to trigger, and will have high consequences. These avalanche will be 2-4 feet deep, hard slabs up to D2 in size, and easily capable of burying, injuring or killing a person. These hard slabs will allow multiple people to travel on slope before someone finds a weak point and triggers an avalanche. The distribution of the persistent slab problem exists in portions of terrain on all aspects at all elevations. This problem has a high variability and continues to be hard to assess and predict.

New problem: The slab problem closer to the surface will be shallower and highly variable in depth, 4 inches to 2.5 feet deep. These slabs were formed by the wind. Expect to find them on most leeward aspects, W to NE, at mid and upper elevation, and reactive on slopes above 35º. These will also be hard slabs, but smaller in volume and destructive force. Some may be large enough to bury, injure or kill. Be extremely cautious of these shallower slabs at and near ridgelines and in places that have the added consequences of terrain traps, such as cliffs, rocks and depressions. It will also be possible to trigger these types of avalanches mid-slope. Be cautious of steep roll-overs and unsupported slopes. Larger, uniform slopes will also have the potential for more continuity, making the slab width larger and more difficult to escape. It may be possible for these slabs to step down into the basal facets, increasing the size and volume of the avalanche and it’s consequences. It will be challenging to see visual clues of past wind loading with the overnight blanket of new snow.

Both of these persistent slabs will be possible to trigger from below; avoid traveling in the runnout of slopes above you.

Travel Advice: Heightened avalanche danger on specific terrain features. Identify features of concern and evaluate snow and terrain carefully. Rapid weather changes will increase the avalanche hazard quickly.

Video of the deeper, persistent slab problem:

Notice that it takes significant force to get the weak layer to fail, but once it does, the propagation and energy are high.

While the deeper problem may be more consequential, you are more likely to trigger the shallower persistent slab problem sitting on old, near surface facets, in the mid-pack.

Feb 17 – X4068, N, 3500′, 35-40 deg start zone, rollover, mid-slope. Human triggered avalanche on Friday, shallow to 2 foot crown, averaging 6 inches. 200 feet wide, 200 feet long. Skier triggered and skied off slab. High propagation. High spatial variability will limit the size of these avalanches.  Hard slab, pencil to knife hard over 4F near surface facets.

## Recent Avalanche Activity

A natural wind slab was observed on a NE aspect of 4068′, Whimpy’s Bowl , likely on 2/15 after a strong wind event. Observation was made from afar after a couple inches of new snow covered the old slab.

On 2/17, a human triggered avalanche occurred on a 35°, N aspect, 3700′ below peak 4068′. The avalanche was 200ft. x 200 ft. The crown varied from 4 inches to 2 ft. thick. The persistent slab (old wind slab) failed on buried near surface facets.  HS-ASu-D1.5-I This is a great example of the persistence of these types of weak layers…in the absence of recent avalanche evidence, avalanches are still possible.

## Recent Weather

This week’s weather at 3550′:

2-4 inches of new snow accumulated through the week, and before Friday.

Below zero and single digit temps early in the week, rose into the mid to upper twenties later in the week, with a high of 36° F

Overnight at 3550′:

5″ new low density snow / 0.3″ SWE overnight

8″ new low density snow / 0.4″ SWE since Friday morning.

This week’s weather at 4500′:

Temperatures this week started cold, averaging 3° F 2/10-2/12, then rose into the teens and 20’s F. Temps averaged 14° F.

Winds averaged 10 mph , gusts 17 mph SE.

Winds gusted 40-50 mph SE for 7 hours on 2/14 .

Overnight at 4500′:

Temps averaged 14ºF, Low of 13° F and a high of 15° F.

Winds averaged 5 mph SSE, gusting 15 mph.

The avalanche hazard will remain the same through the weekend.

Any rapid weather change will easily increase the avalanche hazard.

NWS rec forecast here

State Parks Snow Report for Feb 17, 2017 here

Posted in HPAC Forecasts.